Explain the use of the GROUP BY clause in SQL.

The GROUP BY clause in SQL is used to group rows that have the same values in specified columns into aggregated data.

In more detail, the GROUP BY statement is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns. This means that it combines rows with similar values into larger sets that can be viewed and manipulated as a single unit. This is particularly useful when you want to perform calculations on groups of data within your database.

For example, imagine you have a database of students and their test scores. You could use the GROUP BY clause to group all the students by their score, allowing you to quickly and easily see how many students received each possible score. Or, you could group by the student's year group, allowing you to calculate the average score for each year group.

The GROUP BY clause is not just limited to one column, you can also group by multiple columns. For instance, in the students database, you could group by both the year group and the test score. This would allow you to see the distribution of scores within each year group.

The syntax for the GROUP BY clause is: SELECT column_name(s), aggregate_function(column_name) FROM table_name WHERE condition GROUP BY column_name(s). The aggregate functions you can use with GROUP BY are COUNT, SUM, AVG, MAX, or MIN.

It's important to note that the GROUP BY clause must follow the WHERE clause in a SELECT statement and precede the ORDER BY clause if one is used. Also, every column listed in the SELECT clause that is not an aggregate function must be included in the GROUP BY clause.

In conclusion, the GROUP BY clause is a powerful tool in SQL that allows you to group and aggregate your data, making it easier to analyse and understand.